The English National Opera seemed to be heading for further chaos last night following the shock resignation of its artistic director and chief executive, Sean Doran. Dogged by a series of crises since his appointment in April 2003, Doran stepped down on Monday, apparently without giving notice to members of the company.

His resignation, which is to take effect immediately, followed rumours of clashes with the ENO's vice-chairman Vernon Ellis. According to a source at the ENO, following a board meeting three weeks ago Ellis, chairman of consultancy group Accenture, has been taking a "close interest" in the management of the company and has been at the Coliseum most days.

"The speed of Sean's resignation has come as a shock, but not the inevitability of it," said the source, who asked not to be named. "In the end I think it was a mutual decision."

Martin Smith, chairman of ENO, said Doran, who will continue to work for the company as artistic consultant, had achieved a lot. "The fact that ENO is enjoying a renaissance, with a very successful season artistically and breaking box office records, owes much to the talent of Sean. We are grateful for all that he has achieved at ENO," he said.

Doran said: "While I am sad to be leaving, I am proud of what I have achieved. I took on this role with a number of big challenges for change that would secure the future of the organisation and these have been met. But Doran's resignation leaves the ENO rudderless at its busiest time of year with lavish productions of Madam Butterfly, Xerxes and Billy Budd, although there is speculation that John Berry, director of opera casting, and the ENO's executive director Loretta Tomasi, could job share.

An Irishman recruited from Australia's Perth Festival, Doran's departure is bound to be seen as a vindication of those who had questioned his suitability to head one of the world's biggest opera companies, as well as a setback for Smith, one of his main supporters. "It's disappointing that once again the performers, who are the people most responsible for making the ENO what it is, are the last to be told what is going on," said Paul Daniel, the ENO's former musical director who has been vocal in his criticisms of Smith and Doran.

He added that it was no secret that Smith had been instrumental in Doran's appointment and said it was a "sign of his undying support" that he had remained at the helm for nearly three years.

Doran's resignation is the latest blow to befall the ENO following three turbulent years, during which the company has lurched from one crisis to another. In 2002 Smith announced his tenure as chairman by sacking the ENO's then artistic director, Nicholas Payne. The following January the company said it was sacking a third of its singers to save money. Then in November 2003, amid a strike by the chorus, the ENO had to be bailed out of near-bankruptcy by the Arts Council after racking up huge losses refurbishing the Coliseum. The £41m rebuilding has continued to be dogged by maintenance problems.

In June Daniel was booed by marketing director Ian McKay during his final performance as conductor after giving an interview to the Guardian in which he claimed Doran no longer had the company's support. Daniel, who is now working with the Hallé Orchestra, said he had meant his remarks to be "charitable".

Although Doran scored some box office hits - notably with Anthony Minghella's production of Madam Butterfly - his critics said he lacked artistic vision.

· Backstory

The English National Opera ran into trouble in 2000 when, eight years after acquiring the freehold to the Coliseum in London, it embarked on an ambitious renovation programme, which ran up huge losses. Cost-cutting resulted in the sacking of 80 staff. In April 2003, Sean Doran (above) became artistic director, but in November the Arts Council of England had to rescue the ENO from near-bankruptcy with a £9m grant, noting that the ENO was worth saving, "but not at any cost".